Problem: Luis has taken 6 tests and his average score so far is 85. If he gets 100, a perfect score, on the remaining 3 tests, what will his new average be?
Explanation: If he gets 100 on the remaining tests, the sum of his scores will be $6 \cdot 85 + 3 \cdot 100 = 810$ His overall average will then be $810 \div 9 = 90$.